JP2000261089A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size of a semiconductor laser and to stabilize the output of the laser, and then to make the oscillation wavelength of the laser precisely tunable, after making the wavelength variable. SOLUTION: In a semiconductor laser, composed of a tapered stripe type semiconductor optical amplifier 10 and a wavelength-selecting element, which returns the light 11 emitted from one end face 10b of the amplifier 10 to the end face 10b by reflecting the light 11 after wavelength selection, a fiber grating 20 that has wavelength-selecting elements, for example, a plurality of refractive index changing sections 20c which are formed at regular intervals in a core 20b and change the selected wavelength according to the intensity of an impressed electric field and causes the light 11 made incident to the core 20b to be refracted by selecting the wavelength used. The electric field is impressed on the refractive index changing sections 20c from an electric field impressing circuit 40 via electrode 31 and 32.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly, to a semiconductor laser having a tapered stripe type semiconductor optical amplifier as a light-emitting source. The present invention relates to a semiconductor laser whose oscillation wavelength is made variable by returning to an optical amplifier.

[0002]

2. Description of the Related Art Conventionally, various attempts have been made to obtain a single-wavelength high-power light beam using a semiconductor. ELEC
TRONICS LETTERS Vo
l.29, No. 14, (1993) pp. 1254-1255 shows one such semiconductor laser.

As shown in FIG. 7, this semiconductor laser has a tapered stripe type semiconductor optical amplifier 1 as a light-emitting source, and the light emitted from the rear end face 1a of the semiconductor optical amplifier 1 is collimated by a lens 2 to be collimated. The light is reflected by the reflection type diffraction grating 3 and returned to the semiconductor optical amplifier 1. In this configuration, when the light 4 whose wavelength is selected by the diffraction grating 3 is returned to the semiconductor optical amplifier 1, the wavelength of the light 4F emitted from the front end face 1b is locked to a single wavelength, and a high output of 1 W or more is obtained. Thus, a light beam of high quality and high output close to the diffraction limit can be obtained.

In this semiconductor laser, the oscillation wavelength can be changed by rotating the reflection type diffraction grating 3 in the direction indicated by the arrow A in FIG. 7 to change the selected wavelength.

[0005] Further, the applicant of the present invention aims at lowering the oscillation threshold current and improving the luminous efficiency.
A semiconductor laser using a birefringent filter as a wavelength selecting means instead of the above-described diffraction grating has been previously proposed (see JP-A-10-190105).

[0006] Furthermore, APPLIED PHISICS LETTERS Vol.7
3, No. 5, (1998), pp. 575 to 577, also shows a semiconductor laser using a fiber grating as a wavelength selection means instead of the above-described diffraction grating. As described above, in a semiconductor laser using a fiber grating or the above-described birefringent filter as the wavelength selection means, the oscillation wavelength can be slightly changed by changing the drive current.

[0007]

The semiconductor laser for selecting the oscillation wavelength by the above-mentioned reflection type diffraction grating can change the oscillation wavelength over a considerably wide range. Due to the mechanical means for rotating the laser, it is easy to increase the size of the device, and it is difficult to precisely tune the oscillation wavelength. In addition, there is also a problem that the output is likely to be unstable due to the displacement of the element due to the presence of a mechanical driving portion.

On the other hand, a semiconductor laser using a birefringent filter or a fiber grating as a wavelength selecting means instead of the above-described diffraction grating does not have a mechanical driving part, and therefore does not have the above-described problem. However, the change in the oscillation wavelength due to the change in the drive current is extremely small, and from a practical point of view, it cannot be said that the wavelength is variable.

The present invention has been made in view of the above circumstances, and in a semiconductor laser having a tapered stripe type semiconductor optical amplifier, the oscillation wavelength is variable, the size is reduced, the oscillation wavelength is precisely tuned, and The purpose is to realize output stabilization.

[0010]

A wavelength tunable semiconductor laser according to the present invention comprises a tapered stripe type semiconductor optical amplifier as described above, and a light emitted from one end face of the semiconductor optical amplifier. In a semiconductor laser comprising a wavelength selection element to be returned to an end face, a wavelength selection element that changes a selected wavelength in accordance with the intensity of an applied electric field is used as the wavelength selection element. Is provided.

More specifically, as the above-mentioned wavelength selecting element, for example, a fiber grating having a plurality of refractive index changing portions formed at equal intervals in a core and reflecting and diffracting the light incident on the core is used. Can be applied.
In this case, the means for applying the electric field can be constituted by a pair of electrodes formed so as to sandwich the refractive index changing portion therebetween, and means for applying an electric field between these electrodes.

A birefringent filter having at least one birefringent element formed of a material having an electro-optical effect (EO effect) as a wavelength selecting element, and reflecting the light passing through the birefringent filter. What consists of a combination with a mirror can also be applied. In this case, the means for applying the electric field includes a pair of electrodes formed so as to sandwich the light passing portion in the birefringent element, and means for applying an electric field between these electrodes. Can be.

The birefringent element is LiNb.
_x Ta _{1 -x} O ₃ (0 ≦ x ≦ 1) or a material doped with at least one of MgO and ZnO can be preferably used. In that case, it is desirable that the pair of electrodes be disposed apart from each other in the c-axis direction of the birefringent element.

Further, as a wavelength selection element, an optical waveguide element having a reflection type grating formed on a substrate having an electro-optic effect and reflecting and diffracting the light incident on the optical waveguide may be used. it can.
In this case, as a means for applying the electric field, the one formed on the substrate with the grating portion interposed therebetween is used.
It can be composed of a pair of electrodes and a means for applying an electric field between these electrodes.

The above optical waveguide element is made of LiNb _x T
a _1-x O ₃ (0 ≦ x ≦ 1) or MgO and Z
The substrate can be formed preferably using a substrate doped with at least one of nO.

[0016]

According to the wavelength tunable semiconductor laser of the present invention, as the wavelength selecting element, one that changes the selected wavelength in accordance with the intensity of the applied electric field is used. By adjusting the wavelength, the selected wavelength changes, and thereby the oscillation wavelength can be changed.

More specifically, when the above-described fiber grating is used as the wavelength selection element,
When an electric field is applied to the grating forming portion, heat is generated and the volume of the grating forming portion changes, and the refractive index also changes due to the thermo-optic effect (TO effect).
The effective grating period changes. Therefore, by changing the intensity of the applied electric field, the effective grating period can be changed, and the selected wavelength, that is, the oscillation wavelength can be arbitrarily changed.

On the other hand, when the above-described birefringent filter is used as a wavelength selecting element, when an electric field is applied to a birefringent element formed of a material having an electro-optical effect, the refractive index of the birefringent element is caused by the electro-optical effect. Changes. Therefore, by changing the intensity of the applied electric field, the refractive index of the birefringent element can be changed, and the selected wavelength of the birefringent filter, that is, the oscillation wavelength can be arbitrarily changed.

In the case where the above-described optical waveguide element is used as a wavelength selecting element, when an electric field is applied to the grating forming portion of the optical waveguide, the effective grating period changes due to the electro-optic effect. Therefore, by changing the intensity of the applied electric field, the effective grating period can be changed, and the selected wavelength, that is, the oscillation wavelength can be arbitrarily changed.

As described above, the wavelength tunable semiconductor laser according to the present invention does not have a mechanism for mechanically driving the optical element in changing the oscillation wavelength. By comparison, the size is sufficiently reduced, the oscillation wavelength can be precisely tuned, and a stable output can be obtained.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a wavelength tunable semiconductor laser according to a first embodiment of the present invention.
This tunable semiconductor laser has a tapered stripe 10a.
And a reflection type fiber grating disposed so that a light beam 11 emitted from a rear end face 10b of the semiconductor optical amplifier 10 is incident on a core 20b.
20; a pair of electrodes 31 and 32 formed on the outer peripheral surface of the reflection type fiber grating 20;
And an electric field application circuit 40 for applying an electric field between them.

As shown in FIG. 2, the semiconductor optical amplifier 10 has a width Wi of 4 μm on the rear end face 10b side of the tapered stripe 10a.
m, the width Wo of the front end face 10c side is 360 μm, and the length L is 1.5
mm. As an example of the semiconductor optical amplifier 10, an n-GaAs buffer layer (Si = 1 × 10 ¹⁸ cm ³ ) is formed on an n-GaAs substrate (Si = 2 × 10 ¹⁸ cm ⁻³ ).
^-3 dope, layer thickness 0.5 μm), n-Al _0.5 Ga _0.5 As
Cladding layer (Si = 1 × 10 ¹⁸ cm ^-3 doped, layer thickness 2.5 μm
m), n-Al _0.25 Ga _0.75 As optical guide layer (undoped, layer thickness 0.05 μm), n-Al _0.05 Ga _0.95 As quantum well layer (undoped, layer thickness 8 nm), n-Al _0.25 Ga
_0.75 As light guide layer (undoped, layer thickness 0.05 μm), p
-Al _0.5 Ga _0.5 As clad layer (Zn = 1 × 10 ¹⁸ c
m ^-3 doped, layer thickness 2 [mu] m), p-GaAs cap layer ^{(Zn = 5 × 10 18 cm} -3 doping, is made by creating a layer thickness 0.3 [mu] m) under reduced pressure MOCVD method is used.

In the tapered stripe 10a, for example, an SiO ₂ film is formed on the cap layer by a plasma CVD method, and the SiO ₂ film is removed by photolithography and etching in a tapered region serving as a stripe. By AuZn / Au, and n-side by AuG
A structure in which an ohmic electrode is formed by e / Ni / Au can be used.

Further, both end faces 10b, 10b of the semiconductor optical amplifier 10 are
c is coated with a low-reflectance coating such that the reflectance seen from the inside is 0.5% or less, whereby the semiconductor optical amplifier 10 becomes a so-called traveling-wave amplifier.

As shown in FIG. 3, the reflection type fiber grating 20 has a core 20b having a higher refractive index embedded in a cladding 20a, and a plurality of refractive index changing portions 20c are formed in the core 20b. Optical fibers formed at intervals. This reflection type fiber grating 20
As an example, cladding outer diameter is 125 μm, core diameter is about 10 μm
By forming an interference fringe on the core 20b of the optical fiber for optical communication by two-beam interference exposure using excimer laser light in the ultraviolet region, the refractive index of the portion of the core 20b irradiated with light is changed (increased). Created. It is considered that this change in the refractive index is caused by the chemical change of germanium oxide doped in the core 20b by irradiation with ultraviolet rays.

The electrodes 31 and 32 are formed on the outer peripheral surface of the clad 20a with the refractive index changing portion 20c interposed therebetween. On the other hand, the electric field application circuit 40 includes a closed circuit 41 having one end and the other end connected to the electrodes 31 and 32, respectively, a DC power supply 42 and a variable resistor 43 arranged in series with each other in the closed circuit 41.
It is composed of

The operation of the wavelength tunable semiconductor laser having the above configuration will be described below. Back end face of semiconductor optical amplifier 10
The light beam 11 emitted from 10b enters the core 20b from the end face of the reflection type fiber grating 20, and propagates therethrough. The refractive index changing portion 20c formed on the core 20b
Constitute a grating (diffraction grating) along the propagation direction of the light beam 11. This grating is
Of the light beam 11 propagating through 20b, only light having a specific wavelength corresponding to the cycle is reflected and diffracted, and fed back to the semiconductor optical amplifier 10.

In the above configuration, the light beam 11 emitted from the rear end face 10a of the semiconductor optical amplifier 10 is originally 800 to 820
Although the light is reflected in the reflection type fiber grating 20 and returns to the semiconductor optical amplifier 10, the light reflected by the reflection type fiber grating 20 returns to a single wavelength (half value) in this wavelength band due to the wavelength selection action of the reflection type fiber grating 20. 0.1 nm full width
Degree). Therefore, the oscillation wavelength of the semiconductor optical amplifier 10 is unified to this wavelength. The light 11F of this wavelength is
The light is amplified while traveling forward in the semiconductor optical amplifier 10 (to the right in FIG. 1), and is emitted from the front end face 10c.

Next, how the oscillation wavelength is changed will be described. When a voltage is applied between the electrodes 31 and 32 from the DC power supply 42, an electric field is applied to the refractive index changing portion 20c of the reflection type fiber grating 20 located between the electrodes 31 and 32. The strength of this electric field can be continuously changed by operating the variable resistor 43.

When an electric field is applied to the refractive index changing section 20c,
The refractive index changing section 20c generates heat, changes the volume of the portion, and changes the refractive index due to the thermo-optic effect (TO effect), thereby changing the effective grating period. Therefore, by operating the variable resistor 43 to change the intensity of the electric field, the effective grating period can be changed, and the selected wavelength of the reflection type fiber grating 20, that is, the oscillation wavelength can be arbitrarily changed.

As described above, the present device can change the oscillation wavelength without using a mechanical drive mechanism, so that the size of the device is sufficiently reduced as compared with a conventional device having such a drive mechanism, and The oscillation wavelength can be precisely tuned, and a stable output can be obtained.

Next, a second embodiment of the present invention will be described with reference to FIGS. In FIGS. 4 and 5, the same elements as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted unless otherwise necessary (the same applies hereinafter).

In the second embodiment, the light beam 11 emitted from the rear end face 10a of the semiconductor optical amplifier 10 is collimated by a collimator lens 53, passes through a birefringent filter 50, and then passes through a condenser lens. The light is condensed by 54 and converged on a mirror 55. The light beam 11 is reflected by the mirror 55, and follows the optical path opposite to the above, and the semiconductor optical amplifier
Feedback to 10 Since the wavelength of the light beam 11 to be fed back is selected to a single wavelength by the birefringent filter 50, the oscillation wavelength of the semiconductor optical amplifier 10 is unified to this wavelength. The light 11F of this wavelength is amplified while traveling forward (to the right in FIG. 4) inside the semiconductor optical amplifier 10 and exits from the front end face 10c.

Next, the wavelength selecting function of the birefringent filter 50 will be described. Each of the two birefringent elements 51 and 52 constituting the birefringent filter 50 constitutes a λ / 2 plate with respect to the single wavelength (here, 810 nm). The transmittance of the P-polarized light component (transmittance per interface) of the light beam 11 at the interface between the birefringent element 51 and the air arranged at the Brewster angle is 100%, while the transmittance of the other polarized light components is 100%. The rate is lower, for example about 30% for the S-polarized component. This is the same for the birefringent element 52.

During one reciprocation of the birefringent filter 50, the light beam 11 reaches the interface between the birefringent element 51 or 52 and air,
A total of eight times are incident at Brewster's angle. At this time, light having a wavelength of 810 nm incident on the first interface in the P-polarized state is always P
Since the polarization state is maintained, the transmittance of the P-polarized component per round trip of the lyo filter is close to 100%, for example, about 100%.
99%. Light having a wavelength other than 810 nm does not become as described above because the direction of the linearly polarized light rotates each time it passes through the birefringent element 51 or 52, and is reflected and cut at each of the interfaces. Therefore, the linearly polarized light beam 11 is incident on the birefringent filter 50 in a P-polarized state, and the light beam is reciprocated therethrough, thereby selecting the wavelength.

Next, how the oscillation wavelength is changed will be described. The birefringent elements 51 and 52 are materials having birefringence and having an electro-optic effect (EO effect).
It is made of LiNbO ₃ doped with MgO. One of the birefringent elements 52 is formed with a pair of electrodes 56 and 57 as shown in a front view in FIG. These electrodes 56 and 57 are formed so as to be separated from each other in the c-axis direction of the birefringent element 52 and to sandwich a portion through which the light beam 11 passes. An electric field application circuit 40 similar to that of FIG. 1 is connected to these electrodes 56 and 57.

The DC power supply 42 of the electric field applying circuit 40
When a voltage is applied between the electrodes 56 and 57, an electric field is applied to a portion of the birefringent element 52 where the light beam passes between the electrodes 56 and 57. The strength of this electric field can be continuously changed by operating the variable resistor 43.

When the above-mentioned electric field is applied, the refractive index of the portion of the birefringent element 52 through which the light beam passes changes due to the electro-optic effect. Therefore, by changing the intensity of the applied electric field, the refractive index of the birefringent element 52 can be changed, and the selected wavelength of the birefringent filter 50, that is, the oscillation wavelength can be arbitrarily changed.

The device according to the second embodiment can also change the oscillation wavelength without using a mechanical driving mechanism, and therefore can be sufficiently miniaturized as compared with a conventional device having such a driving mechanism. Also, the oscillation wavelength can be tuned precisely, and a stable output can be obtained.

Next, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, an optical waveguide device 60 is provided as a wavelength selection device. The optical waveguide element 60 is formed by forming an optical waveguide 62 provided with a reflection grating 63 on a substrate 61. Here, as the substrate 61, a LiNbO ₃ substrate doped with MgO, which is a material having an electro-optical effect, is used.

In this wavelength tunable semiconductor laser, the light beam 11 emitted from the rear end face 10a of the semiconductor optical amplifier 10
Are collimated by the collimator lens 53, then condensed by the condenser lens 54, and converge on the end face of the optical waveguide 62. The light beam 11 enters the optical waveguide 62 from this end face, propagates therethrough in a guided mode, and the reflection type grating 63 reflects and diffracts only a specific wavelength component. The reflected and diffracted light beam 11 is fed back to the semiconductor optical amplifier 10 along an optical path opposite to the above.

Since the wavelength of the light beam 11 to be fed back is selected to be a single wavelength by the reflection grating 63, the oscillation wavelength of the semiconductor optical amplifier 10 is unified to this wavelength. The light 11F of this wavelength is transmitted to the semiconductor optical amplifier 10
The light is amplified while traveling forward (to the right in FIG. 6) and exits from the front end face 10c.

Next, how the oscillation wavelength is changed will be described. A pair of electrodes 65 and 66 are formed on a substrate 61 of the optical waveguide element 60 so as to sandwich a portion of the optical waveguide 62 where the reflection type grating 63 is formed. An electric field application circuit 40 similar to that of FIG. 1 is connected to these electrodes 65 and 66.

The DC power supply 42 of the electric field applying circuit 40
When a voltage is applied between 65 and 66, an electric field is applied to the portion of the optical waveguide 62 where the reflection grating 63 is formed. The strength of this electric field can be continuously changed by operating the variable resistor 43.

When the above-described electric field is applied, the refractive index of the portion of the optical waveguide 62 where the reflection type grating 63 is formed changes due to the electro-optic effect, and the reflection type grating is formed.
The effective period of 63 changes. Therefore, by changing the intensity of the applied electric field, the effective period of the reflection grating 63 can be changed, and the selected wavelength, that is, the oscillation wavelength can be arbitrarily changed.

Since the device of the third embodiment can change the oscillation wavelength without using a mechanical drive mechanism, the device is sufficiently miniaturized as compared with the conventional device having such a drive mechanism. Also, the oscillation wavelength can be tuned precisely, and a stable output can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of a wavelength tunable semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is an enlarged plan view of a semiconductor optical amplifier used in the device of the first embodiment.

FIG. 3 is an enlarged perspective view showing a fiber grating used in the device of the first embodiment.

FIG. 4 is a schematic plan view of a wavelength tunable semiconductor laser according to a second embodiment of the present invention.

FIG. 5 is a front view showing a main part of the device according to the second embodiment.

FIG. 6 is a schematic plan view of a wavelength tunable semiconductor laser according to a third embodiment of the present invention.

FIG. 7 is a schematic plan view showing an example of a conventional tunable semiconductor laser.

[Explanation of symbols]

10 Semiconductor optical amplifier 10a Tapered stripe of semiconductor optical amplifier 10b Back end face of semiconductor optical amplifier 11 Light beam 20 Reflective fiber grating 20a Cladding of reflective fiber grating 20b Core of reflective fiber grating 20c Refractive index change part of reflective fiber grating 31, 32 electrodes 40 Electric field application circuit 41 Closed circuit 42 DC power supply 43 Variable resistor 50 Birefringent filter 51, 52 Birefringent element 53 Collimator lens 54 Condensing lens 55 Mirror 56, 57 Electrode 60 Optical waveguide element 61 Substrate 62 Light guide Waveguide 63 Reflective grating 65, 66 Electrodes

Claims (6)

[Claims] 1. A semiconductor laser comprising: a tapered stripe type semiconductor optical amplifier; and a wavelength selecting element for selecting a wavelength of light emitted from one end face of the semiconductor optical amplifier and returning the light to the one end face. A wavelength-variable semiconductor laser characterized in that a wavelength changing element is used to change a selected wavelength in accordance with the intensity of an applied electric field, and a means for applying an electric field to the wavelength selecting element is provided, the intensity being adjustable. . 2. The wavelength selection element has a plurality of refractive index changing portions formed at equal intervals in a core, and is made of a fiber grating for reflecting and diffracting the light incident on the core, and applying the electric field. 2. The apparatus according to claim 1, wherein the means for performing comprises a pair of electrodes formed so as to sandwich the refractive index changing portion therebetween, and means for applying an electric field between these electrodes.
The tunable semiconductor laser as described in the above. 3. A birefringent filter comprising at least one birefringent element formed of a material having an electro-optic effect, and a mirror for reflecting the light passing through the birefringent filter. The electric field applying means comprises a pair of electrodes formed so as to sandwich a light passing portion in the birefringent element, and means for applying an electric field between these electrodes. 2. The tunable semiconductor laser according to claim 1, wherein: 4. The method according to claim 1, wherein the birefringent element is LiNb _x Ta ₁ -xO.
₃ (0 ≦ x ≦ 1) or a material doped with at least one of MgO and ZnO, and the pair of electrodes are disposed apart from each other in the c-axis direction of the birefringent element. 4. The tunable semiconductor laser according to claim 3, wherein: 5. The optical waveguide device according to claim 1, wherein the wavelength selection element is formed by forming an optical waveguide having a reflective grating on a substrate having an electro-optic effect, and reflecting and diffracting the light incident on the optical waveguide. The means for applying the electric field comprises: a pair of electrodes formed on the substrate with the grating portion interposed therebetween; and means for applying an electric field between these electrodes. Item 4. The wavelength tunable semiconductor laser according to Item 1. 6. The optical waveguide device according to claim 1, wherein the optical waveguide element is LiNb _x Ta _1-x.
O ₃ (0 ≦ x ≦ 1) or MgO and ZnO
6. The tunable semiconductor laser according to claim 5, wherein at least one of the two is made of a doped substrate.